Noise-reducing structure



Jan. 19, 1965 F. A. LOVING, JR

NOISE-REDUCING STRUCTURE 2 Sheets-Sheet 1 Filed Jan. 9, 1962 FIG. 1

INVENTOR FRAQNK ABRAHAM LOVING, JR.

A' BYB ORNEY Jan. 19, 1965 F. A. LOVING, JR

NOISE-REDUCING STRUCTURE 2 Sheets-Sheet 2 Filed Jan. 9, 1962 OF- Q0 00- O! 02 ON 0: OO 0% 00 Oh Ow On O? on FEELEEEEFEEEEEEFEEEEEEELEFEEE OO- CON IN VEN TOR N t United States Patent 3,165,916 NOISE-REDUCING STRUCTURE Frank Abraham Loving, Jra, Wenonah, Ni, assignor to El. du Pont de Nernoiirs and Company, Wilmington, Del., a corporation of Delaware Filed Jan. 9, 1962, Ser. No. 165,162 5 Claims. (Cl. 7335) recently, interest in such noise-reducing facilities has increased because of the expanding use of explosives in nondisruptive operations such as metal forming and processing. Frequently, both the testing and the routine use of explosives .in metalworking advantageously can be carried out close to other facilities; however, these facilities must be protected from missiles arising from the explosion and from blast effects, and the occupants must be protected from objectionable noise associated with the use of explosives.

Principles governing the design of such protective structures have been disclosed. U.S. Patent 2,940,300 teaches that a relationship has been developed which permits calculation of the value of the equivalent hydrostatic pressure which will be generated by detonation of an explosive charge within the protective structure. The patent discloses that the pressure generated is dependent on the kind and weight of the explosive charge and on the volume enclosed by the structure as follows:

W P-K in which P is the equivalent hydrostatic pressure in pounds/square inch which must be contained without exceeding the permissible .strain on the structure, Kis a constant characteristic of the explosive being used, W is the weight of the explosive in pounds, and V is the volume in cubicfeet enclosed by thestructure.

In accordance Withthe above relationship, reduction structural strength of the structure than is permissible for the empty structure itself. Accordingly, this invention provides a noise-reducing structure which comprises a chamber, said chamber having walls of sufficient mass and tensile strength to withstand the equivalent internalghydrostatic working pressure, P, inpound s/square inch, in accordance of the equation:

wherein K is an empirical constant depending upon the explosive used, W is the weight of the explosive in pounds,

in V, the volume, enclosed by a given structure. must be compensated by a reduction in the weight of a given explosive which may be detonated without exceeding a given equivalent hydrostatic pressure, P. Conversely, the weight of the explosive charge which may be detonated without damaging the enclosing structure cannot be increased without a proportionate increase in the volume of a structure having givenstrength characteristics. Frequently, however, there is a desire or a need to increase the weight of explosive charge to be detonated in an existing structure. Likewise, there is a desire to minimize the size and cost of a noise-reducing structure designed to be erected in accordance with the principles ,known heretofore.

ture permits a greater loading of explosive per unit of and V is the volume of the empty chamber measured in cubic feet; a closable access means into said chamber; a noise-muffiing gas-venting means emanating from said chamber; and amass of loose granular solid occupying a segment of said chamber, the height of said segment being about to /2 of the height of said chamber.

The shape and construction of the substantially closed noise-reducing chamber will depend on a variety of factors. Desirably, the structure will be free ofsharp angular-joints which lead to localized concentrations of stress. The preferred design is defined as a chamber, or structure,

having a pair of opposed hemispherical ends which are joined to each other and form a spherical chamber, or are joined to a tubular center section to form a cylindrical chamber with rounded ends. -In thelatter case, the structure may be positioned with its longitudinal axis either horizontally -or vertically, the horizontal position being preferred. a

The structure may be made of any material having relatively high tensile strength andresistance to brittle fracture under the conditions of use. However, considerations of cost, availability, ease of fabrication, and

strength characteristics generally dictate the use of steel as a material of construction. Especially preferred are steels of Type T-l made by the United States Steel Corporation and ASTM Types A212B and A-201. The structure may be madetof a single thickness of steelor may be a sandwich construction consisting, for example, of two sheets of steel .and an interposed layer of concrete. In all constructions, the walls, of .theshell must be capable of withstanding the complex pressure pulse developed by the detonation of the explosive charge within the chamber. Thestrength of the wall required to-withstand the pressure pulses generated by detonation of an-explosive within the structure is a function oflthevolume of the chamber, thepkindof explosivedetonated, the weight of the explosive which is detonated, and theyield-strength of the material of which the chamber is constructed.

The empirical expression erated by detonation of anexplosive charge Within the noise-reducing structure, was derived from measurements of structural deformation generated by detonating various explosives in the center of chambers of several sizes and shapes, as described in Industrial and Engineering Chemistry, volume 49, pages 1744-1746 (1957). The constant K used in this expression depends not only on the strength of the explosive but also on the completeness of reaction for the explosive decomposition under thegstrain to be expected in a the conditions which prevail in the noise-reducing chamber. Thus, the value of K equals2 1O for trinitro- 'toluene (TNT), 1.5 x 10 for pentaerythritol tetranitrate (PETN), and 7X 10 for 40% dynamite.

Although PETN has a higher available explosive energy per unit weight (1300 kcaL/kg.) than does TNT (about 860 kcal./kg.), a higher value of K is used for .TNT than for PETN because of the reaction with air of the initial detonation products of highly oxygen-deficient TNT. The rather low K for the 40% dynamite is ascribed at least in part to the incomplete reaction of the ingredients in the absence of a high degree of confinement such as prevails under the usual conditions of use in a borehole.

The values of K given above have'been corrected to allow for dynamic loading but contain no safety factor. A safety factor, however, is included in the calculations which were used in establishing the maximum permissible weight of an explosive to be detonated in a noise-reducing structure of the prior'art." e

The permissible equivalent hydrostatic pressure, P,was established on the same basis as is used for determining the maximum working pressure for conventional pressure vessels. Thus, for a spherical vessel, the ALS.M.E. Boiler and Pressure Vessel Code, Section VIII, Subsection C, 1959 Edition, p. 79, Table UCS-23, specifies. for

a typical structural steel (S201 A).a.maximum allowable stress'of 13,750 pounds per square inch which is 25 percent of the specified minimum tensile strength of this alloy. Said maximum allowable stress in the wall of a spherical vessel will produce a strain (measurable with conventional strain gages) of 310 microinches per .inch, in accordance with standard methods of stress anal .ysis' (see Timoshenko, S. and Woinowsky-Krieger, S., Theory of Plates and Shell, Second edition, page 482,

McGraw-Hill Book Co., New York, 1959). Hence, a measured maximum strain of about 310 microinches per- ;inch Was accepted as the permissible strain limit in a structure made of the specified steel, and the permissible explosive-load limit, correspondingly, was that weight of explosive which produced the permissible limit strain in thenoise-reducing sphere. A series of such strain measurements relating the volume of a noise-reducing structure, the. permissible strain limit, and the weight of the explosive which was detonated established a value for .K which was characteristic for each given explosive. Having thus experimentally established the relationship,

and; determined values of K for several common explosives, said relationship was usable in the design of noisereducing structures of other sizes, shapes, and other materials of construction. Use of the relationship also permitted calculation of permissible load limits for different explosives in any given noise-reducing structure, and also given structure upon detonation of selected explosive charges.

The accompanying FIGURE '1 depicts, partially in section, a particularly preferred embodiment of thisinvention. In this figure, 1 represents the steel shell, spherical in shape;'2 represents an access door, preferably inwardly opening; 3 represents a supporting cradle, also of steel; 4 represents a supporting base, for example, reinforced concrete; 5 representsa steel vent-tube welded to the shell and connected to'a gas mufiler 6 by flanges 7 '-which have between them a ste'el'grating8 which sup- .ports the filling of lightweightichain 9 which provides a devious path for the explosion gases as theyjpass through mufiler 6. Inside the shellis alayer of dense granular or particulate material 10, 'for example sand 'or crushed limestone; Not shown are retaining hooks weld .ed to shell l'to position explosive charges within the V shell and electrical conduits for ignition circuits and instrument circuits. Additionally, other openings with suitable closures may be included for making photographic studies, for forced ventilation, and the like. I

The use of sand as the granular material has been indicated because of its ready availability, low cost, and

are held on a No. 100 sieve (0.0058-inch opening) andall of which pass a sieve having a flt-inch opening.

While positioning of the granular material in other locations may be feasible, I prefer to place the granular material in the bottom of the structure where it can be used as a support for the explosive-bearing assembly and where gravitational forces will keep the material in substantially a fixed and uniform location.

T o minimize the creation of undesirable noises in venting the explosion-generated gases from the chamber, a

mufiling device preferably is incorporated in the vent.

In the specific structure illustrated, chain in a cylindrical .housing is used to create a devious path for the gases.

Any other arrangement such as is conventional in the sound mufliing art to provide a devious path for the gas flow and to attenuate the sound may be used.

FIGURE 2 illustrates the strain-time relationships which resulted from detonations within a noise-reducing structure of FIGURE 1 when empty and when different amounts of loose granular solid were present, as described in Example 1.

The following examples are given to further illustrate the instant invention.

' Example 1 A 12-foot sphere was made having a shell wallfof %-inch-thick steel, an access opening approximately 5 feet high and 2 /2 feet widelocated. approximately halfway between the bottom and top of the sphere and fitted with an inwardly opening door, and mounted on top of the sphere a mutller comprising an IS-inch-diameter by 4-foot-long steel cylinder having a steel support grating between the sphere and the lower endof the cylinder, the space above the grating being substantially filled with a packing of lightweight chain. The sphere was supported in a steel cradle on a concrete base.

Calibrated strain gages were mounted'on the external surface of the sphere to measure the strain, i.e., the deformation per unit length producedin the steel shell by the detonation of-12-pound charges of TNT suspended substantially in the middle of the sphere. The gages were connected to an oscillograph which made a record of the variation in displacement with time. 'By use of a calibration factor, the displacement was expressed as strain in microinches/inch. Measurements ,were made both before vand 1 after placing sand in the spherical structure, withresultsas shown in'FIGURE-Z and in the table below. v V 7 FIGURE 2 shows that the maximum strain (M)'in the empty spherical structure (graph A) occurred a few milliseconds after the initial pressure pulse (I). In the presence of a 1-foot segment of sand, however, the maximum strain (M) was reduced (graph B). With approximately a 2-foot segment of sand in the sphere (graph C), the maximum strain coincided with the initial strain. This amount of sand therefore represents substantially the optimum quantity,

the sphere before the reduction in volume is such that i.e., the quantity .of sand whichpermits detonating maximum charges o-fexplosive with- 'out exceeding the permissible strain onthe sphere. The segment occupied by loose granular material can be in- .creased to a thickness equal to one-half the height of 33 strain, reaches the level of the maximum strain produced by detonation of an equivalent charge in the empty spherical structure. The numerical data corresponding to the strain graphs of FIGURE 2 are shown in the table below.

Percent of Maximum Height Spherical Initial Strain (M) Graph of Sand Volume Strain (I) of Steel Segment Occupied in Micro- Sphere in Feet by Sand inches/inch in Microinches/inch The marked reduction in the maximum strain of the steel sphere was accomplishedin spite of a reduction in the free volume of the sphere and without hampering or minimizing the noise-reducing capability of the spherical structure. i

Hence, reduction of maximum strain of the spherical chamber is achieved when from to /2 of the height substantially empty structure the maximum strain on the structure walls does not come with the initial shock wave resulting from the detonation of the explosive but rather occurs when reverberation by internal reflection of the initial shock wave combines reinforcingly with resonance of'the structure. The presence of the mass ofloose granular material apparently reduces both the resonance. and the reverberation, perhaps by energy absorption, and

therefore reduces the maximum strain. If less than /2 of the height of the structure is occupied by loose granular material, very little reduction of. maximum strain is; achieved. On the other hand, when more than /2 of i the height of the structure is occupied by loose granular V material, the reduction in volume results in pressures great enough to overcome the moderating effects of the loose granular material so that the initial pressure pulse I imposes as much strain as would be a delayed but reinof the chamber is occupied by loose granular material,

a preferred quantity being a segment of height equal to to A the height of the chamber- Example 2 v This example illustrates the reduction in strain which is attained when an explosive-containing assembly is detonated directly on a mass of sand inside the noise-reducing structure, instead of being suspended in the free space above the sand.

Amatol was spread uniformly over the surface of a 1- foot x 5-foot rectangular metal plate resting on the surface of a 2-foot-high segment of loose sand contained in the spherical noise-reducing structure described in Example 1. The explosive was detonated and the resulting strain in' the steel structure was measured. The measured strain was much less than that expected on the basis of previous experience and of design principles 4O previouslydisclosed, as shown in the table below.

Explosive Maximum Strain 1 (microinches/inch) Kind Weight, 1b. Calculated Measured 50/50 Amatoli 1.85 34' 80/20 Amatol- 4.56 84 .42 80/20 Arnatol- 17. 44 320 150 The 17.44-pound amatol charge represents about the maximum which can be detonated in the given empty spherical steel, structure without exceeding the permisstand an equivalent internal hydrostatic working pressure,

P, in pounds/ square inch, in accordance with the equation V. wherein K is an empirical constant dependent upon the explosive used, W is the weight of the explosive in pounds}. and V is-the volume of the chamber measuredin cubic-" 7 feet; a'closable access means into said chamber; "gas-venue sible strain. With the sand in: the sphere, however, the.

strain y about. half that anticipated, and hence a; I much larger explosive'charge can'be detonated on the,

mass of loose sandwithout exceeding the permissible strain of approximately 310 microinches/irich.-

Another desirable feature of the improved structure of this invention also is illustrated in this example, viz., the

explosives-bearing assemblies can rest on the sand base.

rather than having to be suspended or mounted near the center of the sphere in order to distribute the strain and avoid local concentrations on limited areas of the internal surface of the structure. I, i j

The discovery that the presence'of a substantial =mass of a looseg'ranular material permitsa greater loading of explosive per 'unit of structural strength is most'surprising, particularly since the volume of the structure. is considerably educed and the pressure resulting from the 'detonationvari'es inversely with .thetvolume'of the struc-' ture. :While I do nota wish to 'bebound by any theory concerning the beneficial effects of the mass of granular material, I believe the explaantion is that in any empty or spherical. I 1 v a 4. The structure of claim 2 wherein the chamber isv tubular with hemispherical'extremities in concave relation ing means emanating from said chamber; and amass of loose granular solid contained within said chamber.

2. A noise-reducing structure comprising a chamber Within which explosives are detonated,.said chamber having walls of sufficient mass andtensile strength to stand'an equivalent internal hydrostatic workingz pres-1 sure, P, in pounds/square inch, in. accordance with the equation 1 I W P-KV wherein K is an empirical constant dependent upon'the explosive used, W is the weight of the explosive in pounds,

andV is the volume of the enclosed chamber measured in cubic feet; a closable access means into. said chamber; gas venting means ernanating from said chamber; and a]; of saida chamber, the height of said"segment'being about to] mass of loose granular 'solidoccupying a segment about V of vthehei'ght 'ofJsaidchamber.

claim 2 wherein thenchamber is 3.. The structure of to each other.

5. The structure of claim 2 whereinsaid granular solid. 2

material is sand. t i References Cited in the file of this patentf UNITED STATES PATENTS g 1,800,234 Tuttle Apr. 14, 2,940,300 Loving Iune*14,*1960 f. 12,960,859 I Loving No 22,1960, I FOREIGN PATENTS Great Britain "Ma fu, 1932 

1. A NOISE-REDUCING STRUCTURE COMPRISING A CHAMBER WITHIN WHICH EXPLOSIVE ARE DETONATED, SAID CHAMBER HAVING WALLS OF SUFFICIENT MASS AND TENSILE STRENGTH TO WITHSTAND AN EQUAVILENT INTERNAL HYDROSTATIC WORKING PRESSURE, P, IN POUNDS/SQUARE INCH, IN ACCORDANCE WITH THE EQUATION 